This invention relates to apparatus for controlling the movement of a movable element between an initial starting position and a final stopping position. More particularly, this invention relates to such an apparatus wherein a signal indicative of the actual velocity of the movable element is compared with a signal indicative of a command or desired velocity for the particular distance remaining to travel, with the difference or error signal being used to control the drive means for the movable element.
U.S. Pat. Nos. 3,954,163 and 4,091,911 disclose closed-loop servo control system for the d-c servo motors used to drive the carriage and print wheel of a serial impact printer. In each of those servo control systems, a command velocity signal is generated for the particular distance to travel of the carriage or print wheel, as the case may be. This command velocity signal is then compared with a signal representative of the actual velocity of the carriage or print wheel. The difference or error velocity signal is used to control movement of the carriage or print wheel during a coarse or velocity mode of servo control. In a subsequent fine or linear mode of servo control, the actual velocity signal is compared with a position signal instead of a command velocity signal, as is conventional.
Although the most prevalent form of print wheel type serial impact printers is the closed-loop d-c servo control system above-described, there also exist serial impact printers which utilize stepper motors for control of the movement of the carriage and print wheel. For the most part, these stepper motor controlled printers are of the open-loop type, i.e., there is no feedback of position or velocity information. Open-loops systems have the disadvantage of requiring relatively powerful and expensive stepper motors to insure stability and accuracy.
The use of closed-loop systems for stepper motors is also well known. However, to date such systems have proven relatively complex and costly.
It would be desirable to provide a relatively simple and low cost closed-loop control system for a stepper motor which would be especially effective for controlling the rotation of a rotary element, such as a print wheel, requiring a relatively high degree of speed in positioning.